Broken Spheres

Abstract

In the introduction to Book IV of his Epitome of Copernican Astronomy, Kepler explained the motivation of his work and how it may be charitably understood as a reaction to (and extension of) Aristotle’s famous astronomy book, On the Heavens. In particular, Kepler defended his own work, saying that he was not motivated by a lust for glory and novelty—as some had charged—but rather by the love of knowledge and truth. He also outlined which aspects of Copernicus’ heliocentric theory he accepted and which he rejected (or modified). Now, in Part I of Book IV, Kepler begins to articulate his understanding of the universe. Straightaway, he questions the Aristotelian division of the universe into two distinct regions—the heavens and the earth—which are marked off by a sphere which is centered on the earth and which extends out to the moon’s orbit. The dissolution of this division is very significant in that it allows Kepler to appeal to physics in understanding astronomy. Prior to Kepler, physics was understood to be the study of the ever-changing properties of sub-lunary things, while astronomy (as a branch of mathematics) was understood to be the study of the immutable, ethereal and divine supra-lunary bodies. In contrast, Kepler will here attempt to articulate the physical (rather than mathematical) principles which govern the motion of the heavenly bodies. Perhaps the most striking feature of the following reading selection is Kepler’s allegorical use of Christian doctrine in understanding the form and the arrangement of the universe. While this approach may seem alien to many modern readers, one must keep in mind what Kepler is trying to do: he is not attempting to reject causal explanations of the observed order of the World altogether; rather, he is attempting to replace Aristotle’s causal explanations—which he found wanting—with something more appropriate. So as you read the following text, you might simply ask yourself whether you find Kepler’s explanations to be more (or less) reasonable than those of Aristotle. Is there perhaps another explanation which is better than either?

For if the spheres were solid, the comets would not be seen to cross from one sphere into another.

—Johannes Kepler